Code quality: File paths (for settings)

Nobody has claimed this yet.

I see a few cases, similar to below. These are inconsistent, redundant and I think these can be improved by

1. leverage more "global" app constants, with a few specific names e.g. `UpdateFile`, `LanguageFile` etc.
1. direct syntax: at least use `Path.Combine`

A few existing examples:

```c#
// UpdateSettings.cs and LanguageModel.cs

// Non-localizable strings: Files
public const string SettingsFilePath = "\\Microsoft\\PowerToys\\";
public const string SettingsFile = "UpdateState.json";
...
var localAppDataDir = Environment.GetFolderPath(Environment.SpecialFolder.LocalApplicationData);
var file = localAppDataDir + SettingsFilePath + SettingsFile;
```

```c#
// ModuleEnablementService

internal static string SettingsFilePath { get; } = Path.Combine(
Environment.GetFolderPath(Environment.SpecialFolder.LocalApplicationData),
"Microsoft",
"PowerToys",
"settings.json");
```

Besides that, it might be good to catch other improvements in specific files, when we encounter them.

Start by reviewing the examples in UpdateSettings.cs, LanguageModel.cs, and ModuleEnablementService. Search the repository for similar settings and file-path construction, then identify which cases fit the proposed consistency improvements. Done means the relevant paths use shared, specifically named constants where appropriate and avoid direct string concatenation.

Written by the indexing model from the issue text.